Cutting tools such as for example those used for the chip removing metal working as a rule consist of a substrate (base body) of hard metal, cermet, steel or high-speed steel having a wear-resistant single-layer or multi-layer coating of hard metallic substances deposited thereon by means of CVD processes (chemical vapor deposition) or PVD processes (physical vapor deposition). Among the PVD processes a distinction is made between different variants, for example cathode sputtering (sputter deposition), cathodic vacuum arc deposition (arc PVD), ion plating, electron beam evaporation and laser ablation. Cathode sputtering, such as magnetron sputtering, reactive magnetron sputtering and high power impulse magnetron sputtering (HIPIMS) and the arc vapor deposition are among the PVD processes which are most frequently used for the coating of cutting tools.
For the milling with coated solid carbide milling cutter, in particular for the face milling of workpieces of steel, it is particular important that the coating has a high adhesion and low surface roughness in order to obtain good milling results and long service lives. The quality of the coating apart from the composition among others very strongly depends on the coating processes used, wherein most of the processes have advantages and disadvantages which come into effect to a different extent depending on the type of tool, the material of the substrate and the field of application.
In the cathodic vacuum arc deposition (arc PVD), an arc melting and evaporating the target material is burning between the chamber and the target. In the process, a big part of the evaporated material is ionized and accelerated towards the substrate, the substrate having a negative potential (bias potential), and is deposited on the substrate surface. The cathodic vacuum arc deposition (arc PVD) is characterized by a high rate of deposition, by dense layer structures, due to a high ionization of the evaporated material, as well as by process stability. Due to the high ionization of the metals, by using the cathodic vacuum arc deposition (arc PVD), a good bonding or adhesion of the deposited layer to the material below is obtained. A substantial disadvantage, however, is the process-dependent deposition of micro particles (droplets) caused by the emission of small metal splashes, the avoidance of which is extremely complex. The droplets lead to an undesirably high surface roughness on the deposited layers. In some applications, this, in turn, leads to an earlier wear of the tool due to increased adherence of the tool surface to the workpiece, increased friction forces and, as a result, increased cutting forces.
In the cathode sputtering process (sputtering), by bombardment with energy-rich ions, atoms or molecules are removed from the target and transferred to the gas phase from which they are deposited on the substrate, either directly or after reaction with a reactive gas. The magnetron-assisted cathode sputtering comprises two major process variants, the classical DC magnetron sputtering (DC-MS) and the HIPIMS process. In the case of magnetron sputtering, the unfavorable formation of droplets in the cathodic vacuum arc evaporation (arc PVD) does not occur. However, in the classical DC-MS the deposition rates are comparatively low in comparison to the cathodic vacuum arc deposition (arc PVD), which implies higher process durations and thus an economic disadvantage.
In the high power impulse magnetron sputtering (HIPIMS), the magnetron is operated at high current densities in the pulsed mode, resulting in an improved layer structure in the form of denser layers, in particular due to an improved ionization of the sputtered material. The current densities at the target in the HIPIMS process typically exceed those of the classical DC-MS. Depending on the material, by means of HIPIMS an ionization of up to 100% of the sputtered particles can be achieved. At the same time, the short-term high powers and discharge current densities, respectively, acting on the target lead to an increased degree of ionization which can alter the growth mechanism and the bonding of the layers to the material below and thus has an influence on the layer properties.
Layers deposited by means of DC-MS and HIPIMS often exhibit noticeable structural differences. DC-MS layers in general grow in a columnar structure on the material below. In the HIPIMS process, on the other hand, fine crystalline as well as granular crystalline layer structures can be achieved, which are characterized by an improved wear behavior and longer service lives, related thereto, in comparison to DC-MS layers. HIPIMS layers are in general harder than the DC-MS layers, but also show disadvantages with regard to their adhesion to many materials below.
Typical peak power densities in a conventional HIPIMS process are in the range of 20 W/cm2. By using special target cooling devices up to 50 W/cm2 can be achieved. The corresponding discharge current densities are thereby in the range of up to 0.2 A/cm2. Theoretically, power densities and hence discharge current densities which are much higher would be possible, however, the mean power which may be applied to a sputtering target is limited by the limited possibilities of cooling the target. Hence, the sputtering power is applied in pulsed form, wherein the pulse duration is chosen to be so short that it does not result in over-temperature due to the mean power acting on the target, whereby the target temperature and the allowed maximum target temperature are very strongly dependent on the target material and its thermal conductivity and mechanical properties. A disadvantage in this respect is that the pulse technology requires extensive technical equipment, since generators are mandatory, which are capable of splitting the power into sputter power pulses over time and space. This is not achieved by conventional generator technology. In the prior art different solutions to this problem are offered, for example in U.S. Pat. No. 6,413,382 and WO 03/006703, which, however, have disadvantages.
WO2012/143087 describes a HIPIMS process which enables to sputter material from a target surface in such a way that the sputtered material is available in an ionized form to high percentage. This is achieved by means of a simple generator, the power of which is distributed to a plurality of magnetron sputtering sources at time intervals, i.e. a sputtering source at a time period is supplied with maximum power while the next sputtering source is supplied with maximum power during the subsequent time period. In this way, high discharge current densities are realized. The pulse power is only temporarily supplied to the individual cathodes so that in the meantime these have the possibility to cool whereby the temperature limit is not exceeded.
WO2013/068080 describes a process for the production of a layer system by means of HIPIMS, whereby by alternatingly applying longer and shorter pulse durations HIPIMS layers are deposited having alternatingly a finer and coarser granularity. Such a layer system having alternating layers is supposed to have good wear properties.